The present invention relates to a combustion-type power tool, and more particularly, to such power tool capable of driving a fastener of driving such as a nail, an anchor, and a staple into a workpiece by igniting a mixture of air and gaseous fuel, which in turn causes a linear momentum of a piston.
Conventional combustion-type power tools are described in U.S. Pat. Nos. U.S. Pat. No. 5,197,646 and U.S. Pat. No. 4,522,162. A conventional combustion-type power tool according will be described with reference to FIGS. 1 through 3. Throughout the specification, the term “upper” and “lower” are used assuming that the combustion-type power tool is oriented in a vertical direction. The combustion-type power tool 1 has a housing 2 constituting an outer frame and including a main housing 2A and a canister housing 2B juxtaposed thereto. The main housing 2A is formed with an exhaust port (not shown). A head cover 3 formed with an intake port (not shown) is mounted on the top of the main housing 2A. A gas canister 4 is detachably accommodated in the canister housing 2B. The gas canister 4 contains therein a combustible liquidized gas and has a gauging section 4A and an injection rod 4C extending therefrom.
A handle 5 extends from a side of the canister housing 2B. The handle 5 has a trigger switch 6. A magazine 7 and a tail cover 8 are disposed below the housing 2. The magazine 7 is adapted for containing therein nails (not shown), and the tail cover 8 is adapted for feeding the nail in the magazine 7 and setting the nail to a predetermined position. A push lever 9 is movably provided at a lower end of the main housing 2A. The push lever 9 has a tip end adapted to be pressed against a workpiece 40, and has an upper end portion associated with a link member 11 fixed to a combustion chamber frame 10 described later. A compression coil spring 30 is interposed between the link member 11 and a cylinder 20 (described later) for normally urging the push lever 9 in a protruding direction away from the head cover 3.
When the housing 2 is pressed toward the workpiece 40 while the push lever 9 is in abutment with the workpiece 40 against a biasing force of the compression coil spring 30, an upper portion of the push lever 9 is retractable into the main housing 2A.
A cylinder head 12 is secured to the top of the main housing 2A for closing the open top end of the main housing 2A. The cylinder head 12 supports a motor 13 at a position opposite to a combustion chamber 23 described later. Further, an ignition plug 14 is also supported to the cylinder head 12 at a position adjacent to the motor 13. The ignition plug 14 has an ignition spot exposed to the combustion chamber 23. The cylinder head 12 has a gas canister side in which is formed a fuel injection passage 12a which allows a combustible gas to pass therethrough. One end of the fuel injection passage 12a serves as an injection port that opens at the lower surface of the cylinder head 12. Another end of the fuel injection passage 12a constitutes a gas canister connecting portion which is fluidly connected to the injection rod 4C.
As shown in FIG. 2, the cylinder head 12 is formed with a disk like annular groove 12b having an end face 31. The end face 31 defines an upper contour of the combustion chamber 23. The cylinder head 12 has a disk like annular surface 12C surrounding the groove 12b. A motor boss 32 in which a motor 8 is stored protrudes from the end face 31 toward the combustion chamber 23. An ignition ground holding portion 33 protrudes from the end face 31 and extends in a generally radial direction. An ignition ground 34 is attached to the ignition ground holding portion 25 at a position in confrontation with the ignition plug 14.
The ignition plug 14 is ignitable upon manipulation to the trigger switch 6 and upon movement of the combustion chamber frame 10 to its predetermined position because of the pressing of the push lever 9 against the workpiece 40. The motor 13 has a fan shaft 13A, and a fan 15 positioned in the combustion chamber 23 is fixed to a tip end of the fan shaft 13A.
A head switch (not shown) is provided in the main housing 2A for detecting an uppermost stroke end position of the combustion chamber frame 10 when the combustion-type power tool 1 is pressed against the workpiece 40. The head switch can be turned ON when the push lever 9 is elevated to a predetermined position for starting rotation of the motor 13.
The combustion chamber frame 10 is provided in the main housing 2A and is movable in the lengthwise direction thereof. The combustion chamber frame 10 is moved interlockingly in accordance with the movement of the push lever 9, since the lower end portion of the combustion chamber frame 10 is connected to the link member 11. The cylinder 20 is fixed to the main housing 2A. The combustion chamber frame 10 has an inner surface in sliding contact with the cylinder 20. Thus, the cylinder 20 guides movement of the combustion chamber frame 10. The cylinder 20 has an axially intermediate portion formed with an exhaust hole 20a. An exhaust-gas check valve (not shown) is provided to selectively close the exhaust hole 20a. 
A piston 21 is slidably and reciprocally provided in the cylinder 20. The piston 21 divides an inner space of the cylinder 20 into an upper space above the piston 21 and a lower space below the piston 21. Further, a bumper 22 is provided on the bottom of the cylinder 20. The bumper 22 is made from a resilient material. When the piston 21 moves to its bottom dead center, the piston 21 is abuttable on the bumper 22.
As shown in FIG. 3, when the upper end of the combustion chamber frame 10 abuts on the cylinder head 12, the cylinder head 12, the combustion chamber frame 10, and the upper cylinder space above the piston 21 define in combustion the combustion chamber 23.
As shown in FIG. 3, when the upper end of the combustion chamber frame 10 is separated from the cylinder head 12, a first flow passage 24 in communication with an atmosphere is provided between the combustion chamber frame 10 and the cylinder head 12, and a second flow passage 25 in communication with the first flow passage 24 is also provided between the combustion chamber frame 10 and the upper end portion of the cylinder 20. These flow passages 24, 25 allow a combustion gas and a fresh air to pass along the outer peripheral surface of the cylinder 20 for discharging these gas through the exhaust port (not shown) of the main housing 2A. Further, the above-described intake port (not shown) of the head cover 3 is formed for supplying a fresh air into the combustion chamber 23, and the exhaust hole 20a is adapted for discharging combustion gas generated in the combustion chamber 23.
A plurality of ribs 10A protrudes radially inwardly from the portion of the combustion chamber frame 10, the portion defining the combustion chamber 23. Each rib 10A extends in the axial direction of the combustion chamber frame 10. The ribs 10A promote stirring and mixing of the air and the combustible gas in the combustion chamber 23 in cooperation with the fan 15.
Rotation of the fan 15 performs the following three functions. First, the fan 15 stirs and mixes the air with the combustible gas as long as the combustion chamber frame 10 remains in abutment with the cylinder head 12. Second, after the mixed gas has been ignited, the fan 15 causes turbulent combustion of the air-fuel mixture, thus promoting the combustion of the air-fuel mixture in the combustion chamber 23. Third, the fan 15 performs scavenging such that the exhaust gas in the combustion chamber 23 can be scavenged therefrom and also performs cooling to the combustion chamber frame 10 and the cylinder 20 when the combustion chamber frame 10 moves away from the cylinder head 12 and when the first and second flow passages 24, 25 are provided.
A driver blade 26 extends downwards from a side of the piston 21, the side being at the cylinder space below the piston 21, toward the lower end of the main housing 2A. The driver blade 26 is positioned coaxially with the nail set in the tail cover 8, so that the driver blade 26 can strike against the nail during movement of the piston 21 toward its bottom dead center. When the piston 21 moves to its bottom dead center, the tip end of the driver blade 26 strikes against the nail, and the piston 21 abuts on the bumper 22 and stops. In this case, the bumper 22 absorbs a surplus energy of the piston 21.
Next, operation of the combustion-type power tool 1 will be described. In the non-operational state of the combustion-type power tool 1, the push lever 9 is biased away from the cylinder head 12 as shown in FIG. 1 by the biasing force of the compression coil spring 30, so that the push lever 9 protrudes from the lower end of the tail cover 8. Thus, the uppermost end portion of the combustion chamber frame 10 is spaced away from the cylinder head 12 because the link member 11 connects the combustion chamber frame 10 to the push lever 9. Further, a part of the combustion chamber frame 10 which the part defines the combustion chamber 23 is also spaced away from the top portion of the cylinder 20. Hence, the first and second flow passages 24 and 25 are provided. In this condition, the piston 21 stays at its top dead center in the cylinder 20.
With this state, if the push lever 9 is pushed onto the workpiece 40 while holding the handle 5 by a user as shown in FIG. 3, the push lever 9 is moved toward the cylinder head 12 against the biasing force of the compression coil spring 30. At the same time, the combustion chamber frame 10 which is associated with the push lever 9 through the link member 11 is also moved toward the cylinder head 12, closing the above-described flow passages 24 and 25. Thus, the sealed combustion chamber 23 is provided.
In accordance with the movement of the push lever 9, the gas canister 4 is tiltingly moved toward the cylinder head 12 by way of a cam mechanism (not shown). Thus, the injection rod 4C of the gas canister 4 is pressed against the gas canister connecting portion of the cylinder head 12, so that the combustible liquidized gas in the gas canister 4 is injected into the combustion chamber 23 through the gauging section 4A and the fuel injection passage 12a. 
Further, in accordance with the movement of the push lever 9, the combustion chamber frame 10 reaches its uppermost stroke end whereupon the head switch is turned ON to energize the motor 13 for starting rotation of the fan 15. Rotation of the fan 15 stirs and mixes the combustible gas with air in the combustion chamber 23 in cooperation with the plurality of ribs 10A.
In this state, when the trigger switch 6 provided at the handle 5 is turned ON, spark is generated between the end of the ignition plug 14 and the ignition ground 34 to ignite the combustible gas. The combusted and expanded gas pushes the piston 21 to its bottom dead center. Therefore, a nail in the tail cover 8 is driven into the workpiece 40 by the driver blade 26 until the piston 21 abuts on the bumper 22.
After the nail driving, the piston 21 strikes against the bumper 22, the cylinder space above the piston 21 becomes communicated with the exhaust hole 20a. Thus, the high pressure and high temperature combustion gas is discharged out of the cylinder 20 through the exhaust hole 20a of the cylinder 20 and through the check valve (not shown) provided at the exhaust hole 20a to the atmosphere to lower the pressure in the combustion chamber 23. When the inner space of the cylinder 20 and the combustion chamber 23 becomes the atmospheric pressure, the check valve is closed. Combustion gas still remaining in the cylinder 20 and the combustion chamber 23 has a high temperature at a phase immediately after the combustion. However, the high temperature can be absorbed into the walls of the cylinder 20 and the combustion chamber frame 10. Absorption of the heat into the cylinder 20 etc. causes rapid cooling to the combustion gas. Thus, the pressure in the sealed space in the cylinder 20 above the piston 21 further drops to less than the atmospheric pressure creating a so-called “thermal vacuum”. Accordingly, the piston 21 can be moved back to the initial top dead center position.
Then, the trigger switch 6 is turned OFF, and the user lifts the combustion-type power tool 1 from the workpiece 40 for separating the push lever 9 from the workpiece 40. As a result, the push lever 9 and the combustion chamber frame 10 move away from the cylinder head 12 because of the biasing force of the compression coil spring 30 to restore a state shown in FIG. 1. Thus, the first and second flow passages 24 and 25 are provided. In this case, the fan 15 is configured to keep rotating for a predetermined period of time after the detection of the predetermined position of the combustion chamber frame 10 by the head switch in spite of OFF state of the trigger switch 6. Thus, in the state shown in FIG. 1, fresh air is sucked into the combustion chamber 23 through the intake port formed at the head cover 3 by the rotation of the fan 15. Thus, the combustion gas is urged to flow through the first and second flow passages 24, 25, and is discharged to the atmosphere through the exhaust port formed in the main housing 2A. Thus, the combustion chamber 23 is scavenged. Then, the rotation of the fan 15 is stopped to restore an initial stationary state. Thereafter, subsequent nail driving operation can be performed by repeating the above described operation process.